Fail-operational multiple lifting-rotor aircraft

ABSTRACT

A rotorcraft having multiple rotors, and wings that provide lift in forward flight, has mechanical coupling between rotors that can be disengaged and optionally reengaged, during flight. The coupling can, which can prevent a failure of one rotor from interfering with rotation of the other rotor(s), can be accomplished using many different types of devices, including for example, dog clutches and friction clutches, and collapsible clutches. Disengagement can range from being completely under control of an operator, to partially under operator control, to completely automatic. Among many other benefits, designing, manufacturing, fitting, retrofitting or in some other manner providing an aircraft with a device that can disengage rotation of one of the rotors from that of another one of the rotors during flight can be used to improving survivability in an emergency situation.

This application claims priority to U.S. Provisional Application Ser.No. 60/713,683 filed Sep. 2, 2005.

TECHNICAL FIELD

The field of the invention is multiple lifting-rotor aircraft.

BACKGROUND

Any rotor system, its mounting or controls, or the drive connectionincluding shafting and direction-changing gearboxes can suffer fromimpending or catastrophic failure. The failure cause can be internal,typically a component failure, or external by the sustaining of battledamage. In any event, the method of monitoring the loads, vibrations,temperatures and rotational speeds will determine whether the failure isprogressive or instantaneous, and whether the drive disconnection shouldbe elective or automatic.

When in forward flight, the likelihood of secondary damage to anaircraft structure due to severe vibration from, for example, the lossof a rotor blade must be minimized. Aircraft with multiplelifting-rotors always have a mechanical linkage between the rotors toprevent lift imbalance should one rotor fail to rotate. The problem withthat arrangement, however, is that the safety benefit in vertical flightbecomes a liability in forward, wing-borne flight.

Until very recently, the range/payload capability of a tilt-rotor orcompound aircraft was so limited that attention was not focused on thisproblem. More recently, a combination of gains in aircraft structuralmaterial strength, aerodynamic and dynamic improvements in rotors and inthe fuel efficiency of gas turbine engines has resulted in a substantialimprovement in VTOL aircraft utility. As the utility improves, and asthe scale and payload capability of VTOL aircraft enlarges, (i.e. adecrease in the operating cost per ton-mile), it is clear thattilt-rotor aircraft can become part of a short to mid-rangetransportation system for passengers and freight. Increased attention tothe tilt-rotor aircraft type is based on its potential for reducingairport congestion and at the same time achieving turbo-prop speed andfuel efficiency over ranges of several hundreds of miles. The verticaltake off and conversion phase of a typical flight could now occupy onlyabout one percent of total flight time. Given the mechanical complexityof tilt-rotor aircraft, and the new reality that they are cruise flightrather than hovering machines, a fresh appraisal of the flight safetyimplications of the engines/rotors/drive systems and theirfail-operational behavior is needed. This is the subject and scope ofthe described invention.

In addition, during the last six decades of incremental helicopterdevelopment, design attention was always paid to their flight safety asrotor-borne lifting machines, whether for emergency rescue, the liftingof loads, or for military excursions. A prime objective was to minimizethe effect of power loss leading to the use of two or more engines, andto the permanent shaft connection between the two rotors of a tiltrotor, or the two rotors of a tandem rotor, so that there was no suddenlateral or longitudinal lift imbalance in the event of one enginefailing.

Most tilt-rotor, tilt-wing aircraft and multi lifting rotors compoundhelicopter designs rely on cross shafts between their 2 or 4 rotors toprovide hover lift from all rotors when an engine fails. These crossshafts, in current design practice, are permanently engaged in flightsuch that all rotors turn if any engine is operational. Automaticdisengagement of non operational engines is provided by the use ofone-way clutches (Sprag-type clutches in most cases).

Propeller-driven passenger aircraft, either the piston engine poweredexamples of the 1920-60 era or the modern turboprop powered aircraft,don't hover or VTOL and therefore use no cross shaft. They are capableof safe flight and landing in the case of an engine failure or themechanical or structural failure of a propeller. In such cases thepropeller is stopped and the blades controlled to a streamline position(“feathered” in the aircraft vernacular).

During hover and conversion to forward flight, all known tilt-rotor andtilt-wing aircraft cannot continue flight to a safe landing when a rotor(not an engine) fails. However, as previously noted, these two flightregimes occupy a small and decreasing percentage of the total flighttime as the utility of the aircraft improves and it becomespredominantly a transportation machine and not a lifting device. Inorder to render tilt-rotor and tilt-wing aircraft acceptable for largescale transportation of passengers, it must be possible to continuewing-borne flight, in airplane mode, to a safe landing with a damaged ordisabled rotor, and not to allow this single point failure to undulycompromise flight safety.

Thus, there is still a need for methods and apparatus in which a failureof one rotor in a multiple lifting-rotor aircraft not interfere withrotation of the other rotor(s).

SUMMARY OF THE INVENTION

The current invention provides methods and apparatus in which arotorcraft having a mechanical coupling between rotors can be disengagedand optionally reengaged, during flight, and thereby prevent a failureof one rotor from interfering with rotation of the other rotor(s). Thiscan be accomplished using many different types of devices, including forexample, dog clutches and friction clutches, and collapsible clutches.

In preferred embodiments the rotorcraft is either a tilt-rotorrotorcraft or a tilt wing rotorcraft. In the latter case the tiltingwing may be only that portion of the wing that is outboard of thenacelle. In other embodiments the rotorcraft may have a forward thrustdevice other than the rotors, and the rotorcraft can include atechnology that provides for automatic conversion to wing-borne flight.

Disengagement can range from being completely under control of anoperator, to partially under operator control, to completely automatic.In yet another aspect, it is contemplated that the mechanical couplingbetween rotors can be reengaged during flight.

Among many other benefits, designing, manufacturing, fitting,retrofitting or in some other manner providing an aircraft with a devicethat can disengage rotation of one of the rotors from that of anotherone of the rotors during flight can be used to improving survivabilityin an emergency situation.

Various objects, features, aspects and advantages of the inventivesubject matter will become more apparent from the following detaileddescription of preferred embodiments of the invention, along with theaccompanying drawings in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic in plan view of basic elements of a nacelle oftilt rotor aircraft according to aspects of the present invention.

FIG. 2 is a schematic in plan view of a rotorcraft having left and rightnacelles as in FIG. 1, and mechanically coupled left and right wingmounted proprotors.

FIG. 3 is a schematic in plan view of a compound helicopter having leftand right wing mounted propellers and forward and aft main liftingrotors, all mechanically coupled together.

DETAILED DESCRIPTION Matrix of Flight Condition, Failure Type, andCorrective Action

The following is a matrix of prevailing flight conditions, failure type,the shaft disconnection decision and the resulting flight safetyenhancement applicable to a two-rotor tilt rotor. This is anillustrative example, and the same logic would be applied to the drivearrangements and disconnection provisions for other aircraftconfigurations with different drive arrangements.

Flight Shaft Rotor brake regime Failure type disconnection activatedResult of action VTOL (1) Loss of one engine No No Power loss but nolift imbalance VTOL (2) Loss of one rotor, N/A N/A Possibly Catastrophicrotor control or (depends on possible shaft rotor gear box disconnectand transition to forward flight) VTOL (3) Failed cross-wing Yes No Noimpact apart from driveshaft or related lack of power plant right angleredundancy gearboxes Airplane Loss of one engine No No Power loss but noMode (4) thrust imbalance Airplane Loss of one rotor, Yes Yes forContinue single- Mode (5) rotor control or disabled engined flight withrotor gear box rotor yaw control from rudder trim or rotor cyclic.Runway/ prepared surface landing mandated Airplane Loss of cross-wingYes No No impact except no Mode (6) driveshaft or right- symmetricalthrust angle drive available if operating gearbox(es) single-engined.Airplane Rotor damaged but Yes, Yes, Continue single- Mode,VTOL-functional, followed by followed by engined flight (yaw- revert toshort-term vibration voluntary voluntary controlled) followed VTOL (7)permitted shaft re- de- by VTOL landing engagement activation

The above table shows that four (possibly five) out of the seven failurepossibilities could result in flight continuation to a safe landing ifthe shaft disconnection system was implemented. Whilst the failures ofcases 1 to 7 are not necessarily equal in their probability, it is clearthat there are important safety benefits to be gained from shaftdisconnection, and that this is an innovative and necessary feature forfuture tilt rotor developments.

Method of Implementing Shaft Disconnection/Re-Connection

The system elements required for a shaft disconnection arrangementconsist of the de-coupling devices, the actuators (most likely electricactuators) for providing the de-coupling action, a comprehensive sensorset, a computer-based data assessment and decision/display/responsesystem and a brake or retarding device for the uncoupled parts of therotor system. The system can be completely automatic, completely manual,or anything in between.

Information System: A sensor set consisting of an array of sensors iscritically located at multiple points on the drive system, for example,at shaft bearings, gearboxes, rotor hubs and blade attachments. As ispresently implemented in multi-channel data collection and monitoringsystems, the parameters to be measured could include rotational speed,torque, temperature, vibration and noise.

Failure Detection: Sensor outputs are compared to threshold values.Deviations past threshold values can be either gradual (a trend) orinstantaneous (indicative of a catastrophic failure).

Response Possibilities Resulting From Failure Detection: Automatic orautonomous control strategies allow for a range of responses to failuredetection and for operator intervention or non-intervention. Four ofmany possibilities are presented:

-   -   1) Instantaneous reaction to failure, shaft disconnection with        no operator input.    -   2) Time-delayed shaft disconnection, automatic function unless        over-ridden by operator.    -   3) Operator-elected shaft disconnection decision, to proceed/not        proceed dependent on situation requirements and data        indications.    -   4) Operator-elected shaft re-connection, necessary because of        required VL (Vertical Landing).

Coupling Hardware Required For Disconnection. A detailed description ofthe construction features of a separable-under-load, actuator-operatedcoupling is not included, as the specifics of the coupling do not affectits location in the drive system, the implementation of the control orthe resulting safety enhancement. However, the weight of a coupling thatonly separates the drive with no possibility of re-connection under loadis not a significant factor in the overall drive system weight.

Coupling Hardware, Disconnection Followed By Re-connection Under Load.The short-term re-instatement of a damaged but still-functional rotor soas to make an emergency but successful VTOL landing will require there-connection of the drive when substantial torque is present at thetime of re-engagement. This function requires a clutching device, mostlikely with friction plate elements, with its attendant system weightpenalty. The aircraft designer will consider the safety benefit versusthe weight penalty of catering for Flight Condition/Damage Scenario “7”in the matrix of possibilities outlined above, and decide whether theheavier, re-connectable couplings are warranted.

Drive System Configuration Issues; Maximum Safety Benefit FromDisconnection. A study of the power flow from engines to rotors in FIGS.1, 2 and 3 will indicate that the drive should have the most direct,minimum component, path from the engines to the rotors, and that theshaft disconnection element should isolate the maximum ofinterconnection gearboxes (usually the right-angle drive gearboxes), andthe shaft, bearings and coupling components. This fact willfundamentally influence the drive and gear arrangement of the reductionset between the engine and the rotor. For maximum safety, and to gainthe most benefit from shaft disconnection, drive schemes where theengine powers the cross-shaft by bevel gearing, for example, becomeinvalid.

Applicability to Different Types of Lifting-Rotor Aircraft

There are three categories of rotorcraft that have the capability tosustain forward flight from the lift of conventional wings, yet can takeoff and land vertically. All have rotors or thrusters, wings and a drivesystem connecting the propulsion units to the rotors, yet vary in theirconfiguration. The three categories are the Tilt Rotor Aircraft, theTilt Wing Aircraft and the Compound Helicopter.

A tilt rotor is a fixed-wing aircraft with multiple rotors disposedsymmetrically either side of aircraft center. With the rotor axesvertical the rotors are the sole providers of lift for hover or VerticalTake-Off and Landing (VTOL) and with the rotor axes tilted to thehorizontal, they produce the thrust required for sustained wing-bornflight. The most commonly-quoted example of a tilt-rotor is theBell-Boeing V-22 military aircraft. A variant of the tilt rotor is whentwo pairs of wing/rotor assemblies are arranged in tandem fashion ateach end of the fuselage, becoming a quad tilt rotor.

A tilt wing is a non-fixed wing aircraft, whereby the entire assembly ofwing, rotors (usually four), including their nacelles and engines, cantilt about a horizontal, transverse axis of the aircraft. A flyingexample of the tilt wing was the LTV-Hiller-Ryan XC-142A of 1964. Asbefore, but together with the wing itself, the rotor axes are positionedvertically for VTOL and horizontally for flight in airplane mode. Theterm conversion refers to the transitional phase of flight when lift isshared between the rotors and the wing. The conversion corridor is theterm applied to the boundary limits to the physical values of verticaland horizontal velocity, aircraft or rotors pitch angle, and theproportionality between rotor and wing lift.

A compound helicopter is a wing-equipped helicopter which, in additionto its main rotor(s), is provided with a thrust-producing propeller. Anexample is the Lockheed “Cheyenne” of the late 1960's. Variousarrangements are practical, including configurations with tandem rotorsand two thrust propellers and configurations with tandem rotors whichtilt at least one rotor to provide thrust in wing borne flight. The liftand thrust axes of a compound helicopter are non-tilting. The main rotorlift required for VTOL is reduced in mostly wing-born forward flight toa minimum-power, minimum-drag arrangement by means of blade pitchadjustment.

In FIG. 1 a tilt rotor aircraft 100 is depicted by showing only rotorblades 105, a single nacelle 110, and a wing 120. The basic elements ofthe complete nacelle 110 are the engine 111, the rotor 112, thereduction gearbox 113 and the connection at right-angle or miter gearbox140 to the cross-wing driveshaft 130. As can be seen from theillustration, the drive orientation of the cross wing shaft 130undergoes an angle change of approximately 90 degrees at right-angle ormiter gearbox 140. Shaft disconnection can be accomplished atdisconnection device 115, and this design also includes a rotor brake116.

As discussed above, the disconnection device 115 can be any suitabledevice, including for example, dog clutches and friction clutches, andcollapsible clutches. Selection and implementation of the device 115 isaccordance with the disclosure herein is considered to be well withinthe scope of those of ordinary skill in the art.

In FIG. 2 a dual rotor tilt rotor rotorcraft 100 utilizes two nacelles110 as described above but of opposite hand. The cross wing driveshaft130 is shown, together with a mid-wing gearbox 150 that may be requiredto accept an angle change resulting from wing sweep and/or dihedralangles. The location of the shaft disconnection devices 115 is shown,and the rotor brakes 116 are also indicated. Rotors are shown as 105.

The disconnection device 115 should be viewed as being emblematic of allsuitable devices, including specifically a dog clutch, a frictionclutch, and a collapsible clutch.

In preferred embodiments the rotorcraft is either a tilt-rotorrotorcraft (exemplified by FIG. 2) or a tilt wing rotorcraft (notshown). In the latter case the tilting wing may be only that portion ofthe wing that is outboard of the nacelle. In other embodiments therotorcraft may have a forward thrust device other than the rotors, andthe rotorcraft can include a technology that provides for automaticconversion to wing-borne flight.

At an appropriate location, preferably within the fuselage 160, acontrol box 170 contains electronics that can be used to activate one orpreferably both of the shaft disconnection devices 115 and one rotorbrakes 116. The control box can be operated automatically or under somedegree of operator control. Connections between the control box 170 andthe disconnection devices 115/rotor brakes 116 are not shown as theseare considered to be well within the understanding of the art, whenviewed in conjunction with this disclosure. In yet another aspect, it iscontemplated that the mechanical coupling between rotors can bereengaged during flight, presumably also under control of the controlbox 170.

In the case of a quad tilt rotor (not shown), four nacellessubstantially as illustrated would be arranged in two tandem pairs ofthe same configuration.

In FIG. 3, a compound helicopter 300 has a thrust propeller 305Aattached to each of the nacelles 110. The nacelles 110 include engines111, driveshaft disconnects 115, right angle gearboxes 140, and theother components (not expressly shown) of the nacelles 110 as discussedwith respect to FIG. 1. In addition, helicopter 300 has forward and aftlifting rotors 305B and 305A, which are mechanically coupled to theengines 111 of the nacelles 110 via the four-way driveshaft system 330and a four-way distributing gearbox 350. Brakes 316 are disposed tooperate upon the power train connecting the distributing gearbox 350 tothe lifting rotors 305A and 305B.

Among many other benefits, designing, manufacturing, fitting,retrofitting or in some other manner providing an aircraft with a devicethat can disengage rotation of one of the rotors from that of anotherone of the rotors during flight can be used to improving survivabilityin an emergency situation.

Thus, specific embodiments, applications, and methods have beendisclosed in which a mechanical coupling between rotors can bedisengaged and optionally reengaged, during flight. It should beapparent, however, to those skilled in the art that many moremodifications besides those already described are possible withoutdeparting from the inventive concepts herein. The inventive subjectmatter, therefore, is not to be restricted except in the spirit of theappended claims. Moreover, in interpreting both the specification andthe claims, all terms should be interpreted in the broadest possiblemanner consistent with the context. In particular, the terms “comprises”and “comprising” should be interpreted as referring to elements,components, or steps in a non-exclusive manner, indicating that thereferenced elements, components, or steps may be present, or utilized,or combined with other elements, components, or steps that are notexpressly referenced. Where the specification claims refers to at leastone of something selected from the group consisting of A, B, C . . . andN, the text should be interpreted as requiring only one element from thegroup, not A plus N, or B plus N, etc.

1. In a lifting-rotor aircraft having first and second lifting rotors,each of which is normally powered during flight, a wing that provideslift in forward flight and a mechanical link between the rotors, amethod of accommodating a failure of the first rotor, comprising:providing a device that can commandably de-couple a rotation of thefirst rotor from a rotation of the second rotor during flight of theaircraft; automatically detecting the failure; and automaticallyde-coupling the rotation of the first rotor from the rotation of thesecond rotor.
 2. The method of claim 1, wherein the rotors are tiltrotors.
 3. The method of claim 1, further comprising the step ofproviding the with wings that tilt with the rotors.
 4. The method ofclaim 1, further comprising the step of providing the with a forwardthrust device other than the rotors.
 5. The method of claim 4, whereinthe device comprises a dog clutch.
 6. The method of claim 4, wherein thedevice comprises a friction clutch.
 7. The method of claim 4, whereinthe device comprises a collapsible clutch.
 8. The method of claim 4,wherein the device is commandably operated to disengage the link 9.(canceled)
 10. The method of claim 1, further comprising the step ofautomatically converting the to wing-borne flight.
 11. The method ofclaim 1, further comprising the step of reengaging the link duringflight.
 12. (canceled)